Separation methods for water treatment include a heating or phase-change method, a filtration membrane method, and the like. According to the filtration membrane method, it is possible to obtain the water of quality as desired by controlling the size of the fine pores of the filtration membrane, which advantageously improves the reliability of process. Furthermore, since the filtration membrane method does not require a heating process, it can be advantageously used for water treatment using microorganisms that could be adversely affected by heat.
Among the filtration membrane methods is a method using a hollow fiber membrane module in which hollow fiber membranes are disposed in the form of a bundle. Conventionally, the hollow fiber membrane module is widely used in the field of precision filtration such as preparation of sterile water, drinking water, ultrapure water or the like. Recently, however, the application field of the hollow fiber membrane module is extending to sewage/waste water disposal, separation of solids in sewage disposal tank, removal of suspended solids (SS) in industrial waste water, filtration of stream water, filtration of industrial water, filtration of pool water and the like.
The hollow fiber membrane module may be classified into a submerged module and a pressurized module depending on the operation manner thereof.
The submerged module is immersed in a bath containing fluids to be purified. As a negative pressure is applied inside the hollow fiber membrane, only pure fluid is allowed to permeate the membrane and come into the hollow thereof while the contaminants such as impurities or sludge remains outside the membrane. The submerged module is advantageous in that it can decrease the costs for setting up the facilities and for operating thereof since it does not require any facilities for the fluid circulation.
Continuous water treatment by the hollow fiber membranes, however, causes the fouling of the membranes which significantly decreases the water permeation performance of the hollow fiber membranes. Since the membrane fouling materials contaminate the membranes in various ways, it is required to clean the contaminated hollow fiber membranes in various ways corresponding thereto.
Among the various methods for cleaning the hollow fiber membranes is a aeration cleaning method.
Hereinafter, the aeration cleaning method of the related art will be described with reference to the FIGS. 1 and 2.
FIG. 1 schematically shows the cleaning of an unit module according to the cleaning method of the related art, and FIG. 2 is a top view of the filtering apparatus of the related art.
As shown in FIG. 1, the hollow fiber membrane module 10 comprises a pair of headers 11 and hollow fiber membranes therebetween. During the filtration process, each header 11 is disposed in a direction (Z direction) substantially perpendicular to the surface of the water to be processed, and the hollow fiber membranes 12 are disposed in a direction (X direction) substantially parallel to the surface of the water. Both ends of each hollow fiber membrane 12 is respectively potted in the headers 11 through the fixing parts 11b. The filtrate passing through the hollow fiber membranes 12 is delivered to a filtrate storage tank (not shown) through the outlet port 11a. 
Below the hollow fiber membrane module 10 is an aeration tube 20. The hollow fiber membranes 12 are cleaned as the bubbles ejected from the aeration holes (H) of the aeration tube 20 raises.
As the filtration process is performed for a long time, the contraction of the hollow fiber membranes 12 occurs thereby causing the ends of the hollow fiber membranes 12 connected to the headers 11 to be broken or causing the separation of the hollow fiber membranes 12 from the headers 11. To solve this kind of problem, both ends of the hollow fiber membrane 12 are generally potted in the pair of headers 11 in such a way that there is slack in the hollow fiber membrane 12.
Due to the slack in the hollow fiber membrane 12, however, during the aeration cleaning process, the raising bubbles and water flow make the hollow fiber membrane have a shape convex toward the upper direction.
The bubbles ejected from the aeration tube 20 through the aeration holes (H) thereof hit the hollow fiber membranes 12 having a shape convex toward the upper direction and, as a result thereof, raise gradually toward the middle portions of the hollow fiber membranes 12.
Consequently, as shown in FIG. 2, while the middle portions of the hollow fiber membranes 12 corresponding to the high bubble density area (HA) can be sufficiently cleaned, the both end portions of the hollow fiber membranes 12 corresponding to the low bubble density area (LA) cannot be sufficiently cleaned, which makes the whole filtration efficiency of the filtering apparatus decreased.